The present invention concerns a press roll with controllable deflection for treatment of web type material, specifically for the treatment of a paper web in a press or a calender, where a roll shell is provided which rotatably is connected with a fixed axle that extends through the roll shell and serves as a flexural backing and defines together with said axle a ring-shaped intermediate space. Within the intermediate space a pressure chamber is defined which at least approximately corresponds to the web width of the material, and which is charged with a pressure fluid. The pressure chamber is defined in part by transverse seals in the area of the roll ends, each of which is spaced from the end of the roll shell and extends at least across part of the circumference of the axle leaving between the end of the roll shell and the transverse seal a free marginal zone on both sides of the web-shaped material.
Press rolls of this type are known and, in interaction with backing rolls, serve to treat in a specific way the generally endless web type material that passes between them. In conjunction with paper machines, press rolls of this type are used, e.g., in the wet press or in the calendering sections of press stations. As the width of the paper machines increases, the problem of maintaining a constant line force across the entire width of the paper web assumes decisive importance, and this problem is basically solvable only with a roll whose deflection can be controlled. In the case of such rolls, backing forces which are active in the pressing plane are generated between the rotating shell and the fixed axle. The backing forces between the shell and the axle are generated by one or several hydraulic pressure chambers or pressure elements that are arranged between the shell and the axle. Thus, viewed across the width of the paper machine, the press roll can be adjusted at any time in its entirety or also zonewise to compensate for deflections of the backing roll, making it possible to meet the requirement of a constant line force across the width of the web.
This requirement, however, must be met not only in the center area of the web width, but, ideally, it must be met up to the edge of the material web. This applies especially to calender sections consisting of two or more rolls that are forced on one another and between which passes the paper web that is to be calendered. Naturally, the rolls must accurately fit on one another up to the paper web edge in order for the paper web to leave the calender section with an exactly identical thickness as viewed across the entire width of the calender section.
Insufficiencies still occur in known roll arrangements for the following reasons: On the one hand, if one of the rolls of the calender is heated, i.e., if the temperature of the roll is higher than that of the paper, a heat flow occurs from the roll to the paper. This results in a reduction in diameter of the roll which is non-uniform, as viewed across the entire width of the calender, because the roll edges give off less heat in the area that extends beyond the paper web. The marginal areas of the roll thus have a larger diameter than the area swept by the paper web, and the result is that the edges of the paper web will be squeezed together. Hence, the paper web leaving the calender possesses edge sections that are thinner. On the other hand, if the temperature of the roll is lower than that of the paper, a reciprocal effect occurs. The center area of the roll absorbs heat from the hotter paper, but the marginal area do not. These rather give off heat to the environment, with the ultimate result that the marginal areas remain smaller in diameter than the center area of the roll, which expands due to heat absorption. As a consequence, the paper web leaving the calender is heavier on the edges than it is in the center area.
Balancing the temperatures of the roll and of the paper web is very complex, all the more so as this balancing is subject to environmental effects and strong external effects resulting from interruptions of production. The fit between the roll in the area of its ends, that is, in the area of the paper web edges, also depends ultimately on other factors in addition to the temperature of the paper web and the temperature of the heating fluid in the rolls. The machine speed, moisture of the paper web, tension of the paper web and the wrap angle between rolls and paper web also play a considerable role. A change of the width of the paper web being produced relative to the width for which the calender was conceived also has a considerable influence. Additionally, it must be taken into account that the marginal areas of the rolls are subject to increased wear, particularly in the case where the temperature of the roll is lower than that of the paper web, with the result that over the service life of a set of rolls paper webs will be produced with edges that become increasingly thicker.
These drawbacks can partly be influenced o minimized with expensive and more complex auxiliary devices, such as blowing nozzles, induction coils, etc., for specifically affecting the temperature distribution along the rolls. With the measures proposed so far, however, it has not been possible in all cases to achieve an equalization of the diameter across the entire width of the rolls in such a way that variations in the marginal areas will practically be eliminated and the thickness of the paper produced will be exactly equal across the entire width of the web.
Previously known from the German patent document 31 28 722 (FIG. 9) is a roll with controllable deflection where the temperature distribution is influenced across the width of the roll by means of fluid chambers which zonewise carry different temperatures. In these prior rolls, where a semicircular pressure oil cushion is subdivided in several chambers, the deflection is influenced by variation of the oil pressure and the roll diameter is influenced zonewise by variation of the temperature of the pressure oil. In the prior roll, the width of the area which inwardly is pressurized with pressure oil equals approximately the width of the paper web to be treated. The above-mentioned drawbacks occur also with these rolls, since the ends of the rolls deform conically toward the end, in the area of the paper web edge. Depending on the temperature gradient between the paper web and the end sides of the rolls, thicker or thinner edges are thus created along the paper web. Furthermore, this prior roll possesses disadvantages insofar as it features transverse seals that slide on the inside of the roll shell for separation of the pressure chambers, as viewed across the width of the roll. Such transverse seals are known to cause localized temperature increases, due to friction on the roll shell, and thus undesirable, non-uniform diameter increases of the roll.
Aside from the roll known from the German patent document 31 28 722, press rolls of that type are known from "Das Papier," 1986, page 485-495, and "Das Papier," 1988, page 325-330.
The former paper ("Das Papier," 1986) presents a press roll where the ring-shaped intermediate space is partitioned by means of two longitudinal seals so as to obtain a semicircular pressure space for transferring the pres force from the roll shell to the flexural backing. The two longitudinal seals are arranged on the yoke and extend from the one transverse seal to the other transverse seal. According to the one embodiment, the transverse seals extend only approximately across one-half of the circumference of the flexural backing, so that only a single, semicircular pressure chamber exists (compare FIG. 8). According to a second embodiment, the transverse seals extend across the entire circumference of the flexural backing, thereby forming a semicircular high-pressure chamber and an opposite semicircular low-pressure chamber that acts in the opposite direction (compare FIG. 6).
The second paper ("Das Papier," 1988) describes a press roll where the transverse seals extend across the entire circumference of the bending entity and where no longitudinal seals exist. The pressure chamber is defined solely by the transverse seals and, thus, is ring-shaped. Hydrostatic backing elements are provided in the pressure chamber for transferring the press force from the roll shell to the flexural backing. What has been said for the German patent disclosure 31 28 722 is basically applicable to the press roll taught here. A sufficiently wide marginal area is missing between the inside of the press rolls and one edge of the paper web in both, and the same problems occur.
Based on the above prior art, the problem underlying the invention is to so advance and improve a press roll of the general type that thermally caused diameter differences between these marginal areas and the center area of the roll shell will be avoided.